The present invention relates in general to clutches in which the friction surfaces work in a hydraulic fluid, for example oil, the control of the engagement of the clutch being hydraulic. More particularly, it relates to a hydraulic clutch of the kind which can be conveniently incorporated with a torque converter or similar fluid coupling of a hydraulically controlled automatic transmission, particularly for an automobile vehicle, and of the kind which includes a set of two pistons and a spring interposed therebetween so that the engagement of the clutch is smooth and reliable.
As is well known, there are two types of hydraulic clutches, one type of which includes a single piston slidably received within a casing constituting a cylinder whereby the engagement of the clutch is obtained when a clutch disc is gripped between the single piston and one end of the casing, and the other type of which includes a set of two pistons cooperatively arranged within a casing and a spring interposed between the two pistons whereby the engagement of the clutch is smooth via the effect of the spring which produces a gradually increasing clutch engagement force. It is known in the art that the hydraulic clutch equipped with cooperatively arranged two pistons and a spring (hereinafter referred to as dual-piston hydraulic clutch for brevity) is superior to the clutch with a single piston with respect to smooth and reliable clutch engagement operation. However, the conventional dual-piston hydraulic clutch has suffered from the drawback that its pistons both have very complicated shapes and large dimensions which reduce productivity and thus make same expensive.